I have read that Hoppe's No. 9 on nickel plated firearms.

[stansdds]

Applying a healthy dose of Hoppes No. 9 to the surface and letting it sit for hours could constitute "soaking". As previously stated, copper dissolution is the issue, but S&W did not apply a copper intermediate layer beneath their nickel finishes. Colt and likely others did and there may have been aftermarket refinishers who plated first with copper, then nickel.